Example: AVERAGE

Takes two numeric arguments and returns the mean.

> (define-macro (average num1 num2)

(list '/ (list '+ (eval num1) (eval num2))

2) )

> (macroexpand '(average 2 (* 5 4)))

(/ (+ 2 20) 2)

or, in DrScheme (uses top-level env’t):

(#%app (#%top . /) (#%app (#%top . +) (#%datum . 2) (#%datum . 20)) (#%datum . 2))

> (average 2 (* 5 4))

11

Arguments to macros are not automatically evaluated, as they are in function calls.

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